With the advent of mobile "lap top" computers, there has been an increased demand to link such devices in a wireless local area network. Likewise, there has been a marked increase in the use of wireless devices such as miniature cordless phones and pagers. A general problem in the design of laptops and other types of small, portable, wireless data communication products lies in the type of radiating structure required for the unit, which should be convenient and reliable. When an external dipole or monopole structure is used, such an antenna can be readily broken in normal use. Also, the cost of the external antenna and its associated conductors add considerably to the cost of the final product.
In an effort to avoid use of an external antenna, some manufacturers have used conventional microstrip patch antennas, the characteristics of which are well known. Basically, a microstrip patch antenna comprises a dielectric material, such as a printed circuit board, which has two opposed surfaces. One of the surfaces is coated with an electrically conductive layer which functions as a ground plane, and the other opposed surface has an essentially rectangularly or circularly shaped electrically conductive layer ("microstrip patch") disposed so as to extend over the ground plane. This structure provides the main radiating element of the microstrip patch antenna. In the rectangular patch antenna, the rectangular patch has a length equal to substantially one-half the wavelength of the resonant frequency, also called the resonant wavelength. In the circular patch antenna, the circular patch has a diameter of about 0.6 of the resonant wavelength. Either type of microstrip patch antenna presents a thin resonating cavity wherein a standing electromagnetic wave can exist in the patch and wherein radiation emanates from the edges thereof.
Microstrip patch antennas, however, have many limitations. One limitation is that the microstrip patch antenna can only typically radiate above the ground plane, which is a necessary element of the device. The need for a ground plane also causes the resonant frequency of the antenna to depend on the dielectric constant of the printed circuit board, which can vary considerably due to manufacturing variances. Thus it is difficult to mass produce tuned devices of this kind. Moreover, because the microstrip patch antenna is a highly resonant thin cavity structure, the bandwidth of such an antenna is greatly dependent upon the thickness of the dielectric material. Thus, very thin printed circuit boards, which are increasingly found in portable wireless communications devices, tend to limit the available bandwidth provided with such antennas. Finally, the length of the microstrip patch antenna is relatively large due to the necessity of having an overall length of about one-half the wavelength of the resonant frequency or about 0.6 of such wavelength in the case of a circular microstrip patch antenna. There are methods known in the art for increasing the bandwidth and reducing the size of the microstrip patch antenna, but such method are relatively complicated and generally not conducive to mass produced devices.
The present invention seeks to overcome the limitations of prior art printed antenna structures and allow for more robust radiating characteristics while being more tolerant of manufacturing variances typically encountered in mass produced printed circuit boards. It also desired for the printed antenna to occupy relatively little area on the printed circuit board or other dielectric material.